The invention relates to acoustic transducers such as generally used in high fidelity sound reproduction systems. More specifically, the invention relates to electrostatic or capacitive audio speakers.
Electrostatic speakers are generally of two broad design types. The simplest is a constant voltage design whereby a very thin and tightly stretched diaphragm is placed between two perforated electrodes maintained at constant high voltage. The diaphragm has a conductive coating on each side and receives audio voltage. An air gap is provided between the diaphragm and the electrodes. The sound generated by the diaphragm is transmitted through the air gap and radiates from the open perforations in the electrodes.
Another broad design type for electrostatic speakers includes perforated electrodes that are at a D.C. ground voltage with audio signals applied 180.degree. out of phase and having a tightly stretched diaphragm placed therebetween. The diaphragm has a high resistance coating on each side so that a high D.C. voltage is applied through the large resistance resulting in a constant charge. Two high voltage drive points are required with the constant charge electrostatic speaker design. Only one drive point is required for the constant voltage design
In the design of an electrostatic speaker, certain parameters must be considered. Diaphragm displacement varies greatly with the frequency of the impressed audio signal. For constant radiated audio power, diaphragm is inversely proportional to the square of the impressed audio frequency. At 20 Hz, displacement is 10.sup.6 times greater than at 20 KHz. If for 20 Hz displacement is, for example, 0.1 inches, then at 20 KHz, the displacement would be 0.0000001 inches. At 100 Hz, the displacement would be only 0.004 inches. This displacement is small compared with nearly all the air gap spacing commonly used in electrostatic speakers. Thus, if one is not interested in frequencies below about 100 Hz, the constant charge design and the simple constant voltage design are both suitable.
Powerful amplifiers are required to drive electrostatic transducers. The power required is proportional to the air gap spacing. For maximum output, the transducer operates at a voltage gradient as large as the air in the air gap will withstand without corona or sparking. Thus to minimize the power required, the air gap should be made as small as possible.
The maximum acoustic power available from an electrostatic transducer is limited by the dielectric strength of air. Between the diaphragm and the electrodes, with a 50 mil spacing, a corona will occur at about 5000 volts. This is a gradient of 100 V/mil. Many dielectric materials in thicknesses of a few mils will withstand 5000 V/mil for short time voltage applied. This is about 50 times what air will withstand. For continuous voltage application, many dielectrics will withstand 500 V/mil. This is about 5 times what air will withstand.
Diaphragms in electrostatic transducers are nearly always made of thin plastic film (less than 1 mil). Even a small speak will burn a hole in the film. Electrode design must preclude any sparking at all. This can be achieved by two presently employed techniques. In one technique, the electrode conductors are jacketed by insulation. In a second technique, a relatively thick nonconductive electrode substrate is coated with a conductive coating and a relatively large distance between the diaphragm and the conductive coating on the electrode is provided.
With regard to insulation on electrodes, most common dielectric materials used for electrode insulation have a volume resistivity lying in the range of 10.sup.14 to 10.sup.17 ohms cm. A capacitor made employing such dielectric materials retain its charge for minutes. A difficultly arises when such material is used for insulation on wires or plates to be used as electrodes in electrostatic transducers. When momentary overload voltage gradients in the air gap reach corona level, the charge transfer deposits on the dielectric surface and remains there for some time. This results in reducing the polarizing voltage and reduces the audio output with concurrent sound distortion. As the charge leaks away through the dielectric, the audio output returns to normal.
If the dielectric used in the electrode would have a voltage resistivity of no more than about 10.sup.11 ohm cm, about 10.sup.4 times lower than most insulators, the recovery time would be only about 0.1 seconds, short enough to completely eliminate the problem of recovery time and the concurrent audio distortion. A present technique to reduce the volume resistivity of a dielectric material to 10.sup.11 ohm cm involves adding carbon to an epoxy electrode. However, volume resistivity is hard to control by this method and very sensitive to the amount of carbon present, the mixing time, and other factors. In addition, this material has the undesirable property of being nonlinear; that is, the current through the material is proportional to the square of the applied voltage, and thus the current is not linearly proportional to the voltage applied.
Present designs for electrode structures for electrostatic transducers are of five general types. One present electrode structure employs an insulated wire strung back and forth across a framework providing space between the wires to allow sound to pass through. A second present electrode consists simply of a flat metal sheet with holes in it to allow sound to pass through. A third electrode is simply a flat metal sheet coated with a layer of insulation material. Another present electrode structure is comprised of a sheet of insulating material perforated with a plurality of holes and coated on the outer side with a conductive coating. Finally, another present electrode structure is comprised of a series of relatively thick dielectric bars having a relatively high dielectric constant K and a relatively low volume resistivity. A conductive coating is applied to the outer edges of the dielectric bars.
The insulated wire electrode has poor transient voltage overload recovery and poor performance with a small air gap. The perforated metal electrode is particularly in its resistance to sparking. The perforated metal electrode coated with a dielectric performs poorly with a small air gap, has poor transient voltage overload recovery, and poor resistance to sparking. The electrode comprised of a perforated dielectric with a conductive coating on the outer side has poor resistance to sparking and poor transient voltage overload recovery. The electrode having thick bars of relatively high dielectric constant K and relatively low volume resistivity has excellent transient voltage overload recovery, good spark-free performance, and performs well with a small air gap.
None of the present electrode structures, however, are suitable for speaker designs employing multiple diaphragms. The limited sound pressure available from an electrostatic transducer can be increased markedly by using two or more closely spaced diaphragms suitably driven. Even though the concept of multiple diaphragm transducers has been known for more than 20 years, this method has not been successfully commercialized. If two diaphragms are driven in phase and are to remain in phase at the highest audio frequencies without the need for complicated time-delay electronic circuitry, the spacing between them must be small, on the order of 0.1-0.2 inches. In the actual construction of prior art multiple diaphragm transducers, however, the actual spacing is greater than 0.1-0.2 inches; and time-delay electronic circuitry including complex capacitance and inductance networks is necessary to compensate for an out of phase series of wavefronts presented by a series of diaphragms driven in phase but separated by a larger space.
An insulated wire does not mechanically lend itself to a double diaphragm transducer. A perforated metal or perforated metal coated with a dielectric could be used in a double diaphragm transducer, but acoustic performance would not be good. It would not appear to be possible to fabricate a double diaphragm electrostatic audio transducer from a perforated dielectric having one side coated conductively or using a thick/high-K/high-conductivity electrode having a conductive coating on one side.
Accordingly, it is a primary object of the present invention to provide an electrode for an electrostatic audio transducer having spark free performance.
Another advantage of the present invention is the provision of an electrostatic electrode having rapid recovery from transient electrical overloads.
Another advantage of the present electrostatic electrode is the provision of a uniform electric field.
Another advantage of the present electrostatic electrode is the provision of small air gap spacing between the electrode and the diaphragm.
Another advantage of the electrostatic electrode of the invention is the provision of an electrostatic transducer having multiple diaphragms.
Another advantage of the present invention is the provision of a multiple-diaphragm electrostatic transducer including electrodes having conductors which are capable of being electrically connected in accordance with a plurality of hook-up configurations.
Another advantage of the present invention is the provision of a multiple diaphragm transducer whose multiple diaphragms may be driven in phase without the need for complexity time-delay circuitry to compensate for out of phase wavefronts.
Another advantage of the present invention is the provision of an electrostatic electrode having a volume resistivity of about 10.sup.11 ohm cm and having a linear relationship between current and applied voltage.
Another advantage of the electrostatic electrode of the invention is the provision of an electrode capable of being laminated, non-hygroscopic, and having a dielectric constant of about 10.